﻿using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CC.CRC
{
    /// <summary>
    /// 提供CRC CCITT （ITU)校验码生成和CRC校验功能
    /// </summary>
    public class CRC_CCITT
    {
        // CRC-ITU查找表
        private static UInt16[] crctab16 = new UInt16[]
        {
            0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
            0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
            0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
            0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
            0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
            0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
            0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
            0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
            0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
            0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
            0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
            0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
            0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
            0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
            0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
            0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
            0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
            0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
            0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
            0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
            0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
            0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
            0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
            0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
            0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
            0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
            0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
            0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
            0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
            0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
            0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
            0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
        };
        
        /// <summary>
        /// 计算给定长度数据的16位CRC。
        /// </summary>
        /// <param name="pData">原始数据</param>
        /// <returns>以两个字节返回校验码，低字节在前，高字节在后</returns>
        public static byte[] GetCrcCode(byte[] pData,int startIndex=0,int length=0)
        {
            ushort fcs = 0xffff;    // 初始化
            int k = 0;
            //先初始化为全部计算CRC
            int nLength = pData.Length;
            int start = 0;
            if (length > 0 && length+startIndex < pData.Length)
            {//如果输入了长度，且长度+起始小于数据长度，则按输入的范围计算CRC
                nLength = length;
                start = startIndex;
            }
                
            
            while (nLength > start)
            {
                fcs = (ushort)((fcs >> 8) ^ crctab16[(fcs ^ pData[k++]) & 0xff]);
                nLength--;
            }
            fcs = (ushort)(~fcs);// 取反
            byte[] crc = new byte[2];
            crc[0] = (byte)(fcs & 0x00ff);
            crc[1] = (byte)((fcs & 0xff00) >> 8);
            return crc;
        }
        public static bool IsCrcCheckPass(byte[] pData)
        {
            ushort fcs = 0xffff;    // 初始化
            int k = 0;
            int nLength = pData.Length;
            while (nLength > 0)
            {
                fcs = (ushort)((fcs >> 8) ^ crctab16[(fcs ^ pData[k++]) & 0xff]);
                nLength--;
            }
            return (fcs == 0xf0b8);  // 0xf0b8是CRC-ITU的"Magic Value"
        }
        /// <summary>
        /// 指定起始位置和长度的CRC校验
        /// </summary>
        /// <param name="pData"></param>
        /// <param name="startIndex"></param>
        /// <param name="length"></param>
        /// <returns></returns>
        public static bool IsCrcCheckPass(byte[] pData,int startIndex, int length)
        {
            if (pData.Length < startIndex + length) return false;
            List<byte> list = new List<byte>();
            for (int i = startIndex; i < startIndex+length; i++)
            {
                list.Add(pData[i]);
            }
            return IsCrcCheckPass(list.ToArray());
        }
    }
}